The present invention relates to an apparatus for coating a continuously moving web having the form of a belt-like carrier with various liquid coating compositions, for use in the production of a photographic light-sensitive film, a photographic paper, a magnetic recording tape, an adhesive tape, information recording paper such as a pressure sensitive paper, a thermo-sensitive paper or the like, a light-sensitive printing plate, and so on. The present invention further relates to a method of performing machining on a block constituting the coating apparatus.
A basic structure of an apparatus according to a slide bead system for coating a plurality of layers at the same time with several liquid compositions is disclosed in U.S. Pat. No. 2,761,791. FIG. 5 is a sectional view of a typical coating die used in this field, particularly for coating a web with photographic light-sensitive materials.
A coating composition supplied from a liquid inlet 5 is spread widthwise in a manifold 1 (a portion where the coating composition is spread widthwise in the coating die). Then a layer of the coating composition is formed by a slit 2 (a portion which is on the downstream side of the manifold and where a layer of the coating composition is formed). The formed layer of the coating composition flows on an inclined slide surface 9 and is applied on a web 8 supported by a backup roller 7. In this apparatus it is very important to use a coating die which can give an uniform distribution of the coating quantity.
If the speed of coating is increased or the width of coating is expanded in order to increase the quantity of production, it is inevitable that the quantity of supplied liquid is increased. Accordingly the dynamic pressure of liquid put into the manifold 1 from the liquid inlet 5 is so increased as to give an influence to a flow in the slit. Particularly near the liquid inlet 5, the dynamic pressure becomes so high that the distribution of coating quantity tends to be deteriorated, i.e. the uniformity of the liquid layer is disturbed. This deterioration is conspicuous in the case where the quantity of flow of a coating composition exceeds 0.5 cc/cm.s per unit width.
The influence of the dynamic pressure may be reduced if the clearance of the slit 2 is made narrow to increase the pressure drop. In this case, however, if the mechanical accuracy of the slit clearance is not high enough, there occurs a defect that the unevenness of the slit clearance per se gives an influence directly to the distribution of coating quantity more and more as the slit is made narrower. There is another means in which the liquid inlet 5 is enlarged in order to reduce the dynamic pressure per se. In this case, however, the thickness of a block 6 becomes so large that, in the case of multi layer coating apparatus, there is produced a problem that a coating equipment as a whole becomes large, and at the same time the weight thereof is increased remarkably.
Although a number of patents concerning bead-coating show various shapes of coating dies, most of them adopt a method to supply liquid to a manifold from the center of the die, that is, a center-feed method, and they have potentially the above-mentioned problems.
As a factor which influences the distribution of coating quantity in connection with the fluidity of liquid, there is the pressure drop of a flow other than the above-mentioned dynamic pressure.
In order to eliminate the influence of the dynamic pressure, there is a method of supplying a liquid from a side of the block (a side-feed method) so that the direction of flow in the slit 2 is made perpendicular to the direction of liquid flow the manifold 1. However, the method has a defect in that the uniformity of coating quantity is deteriorated by the pressure drop of the flow in the manifold 1. (This non-uniformity is proportional to substantially the square of the coating quantity.)
Particularly such a problem appears conspicuously in the case where the width of coating is large, for example, beyond 2 meters.
In order to reduce this non-uniformity a method of making the length of a slit shorter in the widthwise flow direction has been used in an casting die for high viscosity liquid, or the like. (FIG. 6) (For example, "Design of Extrusion Die" translated by Tateyo Hayashida, published by The Japan Association of Plastics and Molding Engineers, or "Extrusion Technique for Plastic Sheet" written by Kimimasa Ito, published by Kogyo Chosakai Publishing Co., Ltd.)
This slit length profile indeed has an effect to a casting die for flowing a high-viscosity solution, but in the case of a coating composition of light sensitive material having a comparatively low viscosity (up to 200 4 cp), the distribution of dynamic pressure is not uniform enough when the quantity of flow is increased, so that the distribution of coating quantity is extremely deteriorated. Therefore the profile is not suitable for practical use.
There is a further method of enlarging a hydrodynamically equivalent diameter of a manifold in order to reduce the pressure drop. However, this method increases a liquid reserving volume so that there is a risk of troubles caused by agglomeration of a coating composition, and so on. In addition, it is necessary to make each block thick, so that there is a problem that the weight of the coating apparatus as a whole is increased.